The present invention pertains generally to devices and apparatus for separating different materials from each other according to their respective masses. More particularly, the present invention pertains to electromagnetic devices which employ crossed magnetic and electric fields wherein all of the electric field lines are substantially parallel to each other. The present invention is particularly, but not exclusively, useful as a device for separating charged particles in a multi-species plasma from each other according to their respective cyclotron orbits.
There are many reasons why it may be desirable to separate or segregate mixed materials from each other. Indeed, many different types of devices, which rely on different physical phenomena, have been proposed for this purpose. For example, settling tanks which rely on gravitational forces to remove suspended particles from a solution and thereby segregate the particles are well known and are commonly used in many applications. As another example, centrifuges which rely on centrifugal forces to separate substances of different densities are also well known and widely used. In addition to these more commonly known methods and devices for separating materials from each other, there are also devices which are specifically designed to handle special materials. A plasma centrifuge is an example of such a device.
As is well known, a plasma centrifuge is a device which generates centrifugal forces that separate charged particles in a plasma from each other. For its operation, a plasma centrifuge necessarily establishes a rotational motion for the plasma about a central axis. A plasma centrifuge also relies on the fact that charged particles (ions) in the plasma will collide with each other during this rotation. The result of these collisions is that the relatively high mass ions in the plasma will tend to collect at the periphery of the centrifuge. On the other hand, these collisions will generally exclude the lower mass ions from the peripheral area of the centrifuge. The consequent separation of high mass ions from the relatively lower mass ions during the operation of a plasma centrifuge, however, may not be as complete as is operationally desired, or required.
Apart from a centrifuge operation, it is well known that the orbital motions of charged particles (ions) which have the same velocity in a magnetic field, or in crossed electric and magnetic fields, will differ from each other according to their respective masses. Thus, when the probability of ion collision is significantly reduced, the possibility for improved separation of the particles due to their orbital mechanics is increased. For example, U.S. application Ser. No. 09/192,945 which was filed on Nov. 16, 1998, by Ohkawa for an invention entitled xe2x80x9cPlasma Mass Filterxe2x80x9d and which is assigned to the same assignee as the present invention discloses a device which relies on the different orbital motions of charged particles in a low density environment to separate the charged particles from each other. As implied above, In order to do this the plasma must be generated under low density conditions where the collisionality of the plasma is low. For purposes of the present invention, the collisionality of the plasma is considered to be low when the ratio of ion cyclotron frequency to ion collisional frequency is approximately equal to one, or is greater than one.
As indicated above, plasma centrifuges require a rotational motion of the plasma in order to generate centrifugal forces that are required for separating particles in the plasma from each other. To generate such a motion, centrifuges have typically used an axisymetric radially oriented electric field. However, when ion orbital mechanics, rather than centrifugal forces and particle collisions, are relied on to differentiate particles of different mass, the actual orientation of the electric field need not be so specifically oriented. Consequently, as more thoroughly indicated in the mathematics set forth below, when the collisionality of a plasma is low, charged particles in the plasma, which have different masses, can be distinguished by their cyclotron frequency responses to the magnetic field (e.g. the size of their respective orbits). Importantly, this can be done irrespective of the orientation of the electric field.
The equation of motion of an ion in static electric and magnetic fields is             m      q        ⁢          xe2x80x83        ⁢          r              →        ..              =                    E        →            ⁡              (                  r          →                )              +                  r                  →          .                    xc3x97              B        →            
With a linearly varying electric field
{right arrow over (E)}(x)=Exe2x80x2x{right arrow over (e)}x
(Note that we are measuring the x-coordinate from the line where the electric field vanishes.) and constant magnetic field
{right arrow over (B)}=B{right arrow over (e)}z
the components of the equation of motion (ignoring the trivial z-component) become                     {                                                                                                  m                    q                                    ⁢                                      xe2x80x83                                    ⁢                                      x                    ..                                                  =                                                                            E                      xe2x80x2                                        ⁢                    x                                    +                                                            y                      .                                        ⁢                    B                                                                                                                                                                m                    q                                    ⁢                                      xe2x80x83                                    ⁢                                      y                    ..                                                  =                                                      -                                          x                      .                                                        ⁢                  B                                                                                            {                                                                              x                  ...                                =                                                                            x                      .                                        ⁢                                          xe2x80x83                                        ⁢                                                                  qE                        xe2x80x2                                            m                                                        +                                                            y                      ..                                        ⁡                                          (                                              qB                        m                                            )                                                                                                                                                                y                  ..                                =                                  -                                                            x                      .                                        ⁡                                          (                                              qB                        m                                            )                                                                                                                                            x          ...                =                                            x              .                        ⁢                          xe2x80x83                        ⁢                          (                                                                    qE                    xe2x80x2                                    m                                -                                  Ω                  c                  2                                            )                                =                                    -                              Ω                2                                      ⁢                          x              .                                          
where we have defined       Ω    c    =                    qB        m            ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢              Ω        2              =          -              xe2x80x83            ⁢              (                                            qE              xe2x80x2                        m                    -                      Ω            c            2                          )            
For an ion mass (actually m/q) smaller than a cutoff value       m    c    =            qB      2              E      xe2x80x2      
xcexa9 is real and the orbits are oscillatory. For masses greater than the cutoff they are unbounded. It will be convenient to introduce   δ  =                    (                              m            c                    -          m                )                    m        c              =                  1        -                              mE            xe2x80x2                                qB            2                              =                        Ω          2                          Ω          c          2                    
the fractional mass difference to the cutoff mass.
The complete orbit of an ion with initial position (x0,y0) and velocity (xcexdx0,xcexdy0) is:
x(t)=x0+X(eixcexa9txe2x88x921) y(t)=y0xe2x88x92xcex4xe2x88x92xc2xd(1xe2x88x92xcex4)(x0xe2x88x92X)(xcexa9t)+ixcex4xe2x88x92xc2xdX(eixcexa9txe2x88x921)
with
X=xe2x88x92xcex4xe2x88x92xc2xd(xcexdy0/xcexa9)xe2x88x92xcex4xe2x88x921(1xe2x88x92xcex4)x0xe2x88x92i(xcexdx0/xcexa9)
For bounded orbits, the excursion in the x-direction is 2|X|, and the period in the y-direction is 2xcfx80(xcex4xe2x88x92xc2xd(1xe2x88x92xcex4)(x0xe2x88x92X)). We write out
{dot over (y)}=xe2x88x92xcex4xe2x88x92xc2xd(1xe2x88x92xcex4)(x0xe2x88x92X)xcexa9xe2x88x92xcex4xe2x88x92xc2xdXxcexa9eixcexa9t
for reference
For the special case of an ion initially at rest,
X=xe2x88x92xcex4xe2x88x921(1xe2x88x92xcex4)x0
The excursion in the x-direction is twice the magnitude of this, and the period in the y-direction is 2xcfx80xcex4xe2x88x92{fraction (3/2)}(1xe2x88x92xcex4)x0. Except for the divergence near the cutoff, the fundamental scale of the orbit for any mass is (1xe2x88x92xcex4)x0=mE0/qB2, where E0=Exe2x80x2x0 is the electric field at the initial position.
In light of the above, it is an object of the present invention to provide a linear plasma mass filter which has a substantially rectilinear configuration for its electric field. It is another object of the present invention to provide a linear plasma mass filter which more precisely differentiates between charged particles of different mass (i.e. where the relative mass difference is small). Still another object of the present invention is to provide a linear plasma mass filter which will differentiate between the masses of the charged particles in the plasma. Yet another object of the present invention is to provide for a linear plasma mass filter which is simple effective to use, relatively easy to manufacture, and comparatively cost.
In accordance with the present invention, a linear plasma mass filter includes a container which defines a chamber. For one embodiment of the present invention, the container is shaped substantially like a right rectangular prism. In detail, the container has a first wall which is opposed to, and which is substantially parallel to a second wall. Both the first and second walls are substantially perpendicular to a third wall, and this third wall is opposed to and substantially parallel to a fourth wall. Both the third and fourth walls are, in turn, substantially perpendicular to a fifth wall which is opposed to and substantially parallel to a sixth wall. Stated differently, the container is shaped like a box. In another embodiment, the container may be more cylindrical shaped.
For the above described generally box-like configuration for the container, magnets are mounted on the third and fourth walls of the container for generating a substantially uniform magnetic field (B) in the chamber. Alternatively, current-carrying coils can be wrapped around the third, fourth, fifth and sixth wall of the container to produce a substantially uniform magnetic field (B) in the chamber. In either case, the magnetic field (B) is oriented in the container with its magnetic field lines substantially perpendicular to the first and second walls. Additionally, electrodes are mounted on the first and second walls for generating a rectilinear electric field (E). Specifically, the rectilinear electric field (E) is oriented with its electric field lines substantially perpendicular to the third and fourth walls and generally parallel to the fifth and sixth walls. For this particular configuration, the fifth and sixth walls will be preferably made of a dielectric non-conducting material. Thus, crossed electric and magnetic fields (Exc3x97B) are created in the chamber which act substantially perpendicular to both the fifth and sixth walls of the container. For the cylindrical configuration of the chamber, of course, there will be no separately definable walls. Nevertheless, the functionality of the present invention is not changed so long as there is a electric field (E) in the chamber which is oriented substantially perpendicular to a magnetic field (B) in order that there be crossed electric and magnetic fields (Exc3x97B).
A plasma source provides a multi-species plasma in the chamber, where it is to be processed. As intended for the present invention, the multi-species plasma will include charged particles which have different masses. If, however, the plasma contains some particles that are not single ionized, it is to be understood that the term xe2x80x9cmassxe2x80x9d actually refers to the xe2x80x9cmass-to-charge ratio.xe2x80x9d Accordingly, the multi-species plasma can contain relatively low mass particles (M1) and relatively high mass particles (M2), or even super high mass particles (M3). Importantly, the relatively low mass particles (M1) are responsive to the magnetic field in the chamber by having cyclotron orbits of a first diameter (D1). On the other hand, the relatively higher mass particles (M2) are responsive to the magnetic field by having cyclotron orbits of a second diameter (D2), while super high mass particles (M3) will have cyclotron orbits with a third diameter (D3) which may be infinitely large or unbounded. In this case, due to their different masses, D1 is less than D2, which is less than D3 (D1 less than D2 less than D3).
In order to collect the particles from the multi-species plasma, while they are separated from each other in the chamber, a preferred embodiment for the linear mass filter of the present invention includes a first collector and a second collector. Specifically, the first collector is positioned in the chamber at a height distance above the plasma source, d1. Importantly, the distance d1 is less than the first cyclotron orbit diameter D1 of the lower mass particles M1 (i.e. d1 less than D1). There is also a second collector which is positioned in the chamber at a height distance, d2, above the plasma source. Also importantly, d2 is greater than D1, but less than the second cyclotron orbit diameter D2 (thus: d1 less than D1 less than d2 less than D2). A similar situation results when M3 is also considered.
Because the crossed electric and magnetic fields in the chamber (Exc3x97B) will impart a movement to all of the charged particles (M1, M2 and M3), regardless of their mass, the collectors can be selectively positioned in the chamber to intercept charged particles of a particular mass. Specifically, the particle movement imparted by Exc3x97B will be in the direction of Exc3x97B, which is perpendicular to both the electric field (E) and the magnetic field (B). Accordingly, by positioning the collectors which are intended to intercept the lower mass ions downstream in the direction of Exc3x97B (i.e. the first collector), the second collector can be positioned to intercept the high mass ions as they enter the chamber from the plasma source, without interference from the first collector. The lower mass particles, M1, will, of course, never reach the second collector due to their relatively smaller cyclotron orbit diameters, D1, and will continue to move under the influence of Exc3x97B until they are intercepted by the first collector. Thus, with this configuration, the first collector can be used for collecting the relatively light mass particles (M1), while the second collector is used for collecting the relatively higher mass particles (M2). When this is done, care must be taken to avoid charge build-up that can modify the applied potential.
For one embodiment of the linear plasma mass filter of the present invention, both the first collector and the second collector are plate-like structures which have substantially flat surfaces. For this embodiment, the surfaces of the collectors are parallel to each other and are oriented so that they are substantially perpendicular to the electric field E. The surfaces are also oriented so that they will be substantially parallel to both the magnetic field B and to the crossed electric and magnetic fields Exc3x97B. Recall, the first collector is positioned downstream from the second collector in the direction of Exc3x97B. In another embodiment, the first and second collectors are substantially perpendicular to each other. For this embodiment, a surface of the first collector is oriented substantially parallel to both the electric field E and to the magnetic field B. Also, it is substantially perpendicular to the crossed electric and magnetic fields Exc3x97B. On the other hand, the surface of the second collector is substantially perpendicular to the electric field E and is substantially parallel to the magnetic field B and to the crossed electric and magnetic fields Exc3x97B. For either embodiment, the electric field (E) can be either substantially constant or spatially variable.